1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a facsimile, a printer, etc., and more particularly to an image forming apparatus having a plurality of recording units and a belt to convey a sheet-formed medium.
2. Discussion of the Background
A technology for a full color image forming method to form a full color image by transferring plural color images one after another using a plurality of electrostatic recording units in an electrophotographic color image forming apparatus is described in Japanese Patent Laid-Open Publication No. 10-333398.
A technology for separating a transfer belt from photoconductive elements used for color image formation in a black and white image forming mode to avoid unnecessary depletion of the photoconductive elements is described in Japanese Patent Laid-Open Publication No. 11-95619. A technology for preventing deformation of a transfer belt is described in Japanese Patent Laid-Open Publication No. 11-95516.
The above-mentioned technologies make it possible to prevent deformation of a transfer belt and unnecessary depletion of the photoconductive elements used for color image formation caused by rotating the transfer belt in a black and white image forming mode, in a warm-up state, or in a stand-by position. However, the transfer belt is always in contact with the photoconductive elements used for color image formation in a full color image forming mode. In order to measure a toner density of a developing unit or to detect displacement of toner images among the photoconductive elements, sensor patterns of toner images are formed on the transfer belt by being directly transferred from the photoconductive elements. When the sensor patterns are detected by a sensor disposed at a downstream side of the transfer belt, the sensor pattern, for example formed on the photoconductive element located at the most upstream side, may not be accurately detected due to, for example, a decrease in an amount of adhered toner of the sensor pattern and a mixture of background fouling toner on photoconductive elements with the sensor pattern caused by a reverse transfer of the sensor pattern to other photoconductive elements, and scattered toner in a nip formed between the transfer belt and the photoconductive element because the sensor pattern contacts three other photoconductive elements before the sensor pattern is detected.